As the advancement of electronic industry, various electronic elements now produce more heat than before when they operate. Therefore, natural convection is no longer sufficient for removing waste heat. In order to lower the temperature in a computer system, more than one cooling fan is generally disposed in the computer system to dissipate the heat produced by the electronic elements of the computer system. Normally, the cooling fans are mainly installed close to the heat-generating elements to lower the temperature thereof. Alternatively, the cooling fans are installed at an air inlet or an air outlet to facilitate flowing of air in the computer system. Thus, cooling fans have now become an indispensable part in the computer systems.
Please refer to FIGS. 1 and 2, which are perspective views of two conventional single-rotor blade fans 1. As shown, the single-rotor blade fan 1 has a rotor 10 and a stator 11. The rotor 10 includes a fan blade unit 101 having a row of fan blades spaced on a periphery thereof, whereas the stator 11 has a plurality of stator blades 111, as shown in FIG. 1, or ribs 112, as shown in FIG. 2, spaced on a periphery thereof. With theses arrangements, the single-rotor blade fan causes the air to flow out of the fan blade unit 101 of the rotor 10 at an angle against the stator blades 111, so that the airflow turns to work and the fan 1 provides improved heat dissipation effect. However, the single-rotor blade fan 1, due to its design, has relatively low pressure in the high back pressure zone, and accordingly, has relatively poor heat dissipation effect when being used with high impedance systems.
Please refer to FIG. 3, a fan module 2 is disclosed to include a housing 20, a first and a second set of stator blades 21, 22, a motor 23, and a rotor blade unit 24. The housing 20 has a passage 25, two ends of which respectively form an air inlet 251 and an air outlet 252. The first and second set of stator blades 21, 22 are provided on an inner wall surface of the housing 20 and located in the passage 25. The motor 23 is arranged in the passage 25 and has a rotor 26. The rotor blade unit 24 includes a hub 241, a first and a second set of rotor blades 242, 243. The hub 241 is fixedly mounted around the rotor 26. The first and second sets of rotor blades 242, 243 are circumferentially spaced on the hub 241. The first set of rotor blades 242 is located between the air inlet 251 and the first set of stator blades 21, whereas the second set of rotor blades 243 is located between the first and the second set of stator blades 21, 22. However, the above-mentioned structure does not disclose how the first set of rotor blades 242 is arranged between the air inlet 251 of the housing 20 and the first set of stator blades 21, nor does it disclose how the second rotor blades 243 are arranged between the first set of stator blades 21 and the second set of stator blades 22. Therefore, a person of ordinary skill in the art just could not implement the fan module 2 according to the above-mentioned structure.